Ion thrustor accelerator system

ABSTRACT

A SINGLE COATED GRID ACCELERATOR SYSTEM FOR AN ION THRUSTOR. THE GRID IS COATED WITH AN INSULATING MATERIAL THAT IS BONDED TO ITS SURFACE.

Jan. 5, 1971 B. A. BANKS ET ION THRUSTOR ACCELERATOR SYSTEM Filed Sept.9, 1968 INVENTORS BRUCE A. BANKS SHIGEO NAKANISHI ATTORNEYS UnitedStates Patent-015cc.

3,552,124 ION TI-IRUSTOR ACCELERATOR SYSTEM Bruce A. Banks, NorthOlmsted, and Shigeo Nakanishi,

Berea, Ohio, assignors to the United States of America as represented bythe Administrator of the National Aeronautics and Space AdministrationFiled Sept. 9, 1968, Ser. No. 758,390 Int. Cl. F03h 5/00 US. Cl. 60-2021 Claim ABSTRACT OF THE DISCLOSURE A single coated grid acceleratorsystem for an ion thrustor. The grid is coated with an insulatingmaterial that is bonded to its surface.

STATEMENT OF GOVERNMENT OWNERSHIP The invention described herein wasmade by employees of the United States Government and may bemanufactured and used by or for the Government for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with a singlegrid accelerator system for an ion thrustor. The invention isparticularly directed to a glass coated grid for use in such a system.

Double grid accelerator systems have been used on ion thrustors. Suchsystems combine a screen grid with an accelerator grid. A screen gridincreases the thrustor weight and complexity. Also problems of thermalbuckling and warping have been encountered, and prevention of electricalshorting of the grids presents an insulating problem.

A single grid accelerator system was suggested in an attempt to solvethese problems. Such a system may have boron nitride clamped to theaccelerator grid. It was also proposed to spray aluminum oxide onto theaccelerator grid. It was further suggested that a ceramic material becemented on the accelerator grid.

Various difficulties were encountered in using these grids. Because theboron nitride had to be clamped onto the grid it was in the form ofthick sheet to prevent cracking. The use of a thick insulating coatingreduces thrust because of the increased acceleration distance of theions. The use of thick coatings also limited the size of the ionthrustor 'because as the diameter of the grid increased the insulatorthickness had to be increased to prevent cracking. The flame sprayedaluminum oxide coatings degraded because it is extremely porous tomercury. The ceramic cements are also porous and failed in a mannersimilar to that of the aluminum oxide coatings.

SUMMARY OF THE INVENTION These problems have been solved in theaccelerator system constructed in accordance with the present inventionwhich utilizes a single glass coated grid. The glass forms a strongmechnical bond to a metal grid. When the coating is of the requiredthickness it is electrically reliable.

It is, therefore, an object of the present invention to provide an ionthrustor having an accelerator system which utilizes a single gridthereby eliminating the problems encountered in a double grid system.

Another object of the invention is to provide an improved acceleratorgrid which enhances the ion extraction capability of an ion thrustor.

A still further object of the invention is to provide an improved gridstructure for an ion thrustor which enable 3,552,124 Patented Jan. 5,1971 larger diameter accelerator systems to be used for low voltageoperation.

These and other objects of the invention will be apparent from thespecification which follows and from the drawing wherein like numeralsare used throughout to identify like parts.

DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT In atypical electron bombardment ion thrustor of the type described in US.Pat. No. 3,156,090, mercury vapor is fed into an ionization chamberthrough a distributor plate. Atoms of the vaporized propellant are thenbom- 'barded by electrons emitted from a cathode. The path lengthtraveled by electrons going from this cathode to an anode is greatlyincreased by an axial magnetic field thus causing more ionization byelectron bombardment.

Such a conventional ion thrustor utilizes a double grid acceleratorsystem having 'both a screen grid and an accelerator grid. The screengrid serves to contain the discharge plasma while forming the necessaryion optics to prevent direct impingement of accelerated ions onto theaccelerator grid. Ions in the near vicinity of the screen grid have ahigh probability of being accelerated through openings in the screen andthe accelerator grids because of the high electric fields present at thegrids. Thrust is produced as these ions accelerate through the gridsystem.

Referring now to the drawing an ion thrustor shown in FIG. 1 has anionization chamber 10 for containing a propellant that has been ionizedin any conventional manner. By way of example, this propellant may bemercury that is ionized by electron bombardment as described in theaforementioned US. Pat. No. 3,156,090.

The ion thrustor further includes an accelerator system 12 constructedin accordance with the present invention for accelerating propellantions in the direction of the arrows in FIGS. 1 and 2. The acceleratorsystem .12 utilizes a single grid 14 of an electrically conductingmaterial that is connected to a source of electric power, such as abattery, as shown in FIG. 1. This electrical power source impresses apotential on the grid 14 that is highly negative relative to theionization chamber 10.

An insulator 16 having an annular configuration extends about theperiphery of the ionization chamber 10. The insulator 16 serves toelectrically isolate the single grid 14 from the metal housing formingthe ionization chamber 10.

An important feature of the invention is that the electricallyconducting grid 14 is coated with an insulating material 18. Thisinsulating coating 18 is bonded to the grid 14 as best shown in FIG. 2.

In a conventional double grid system of the type previously described, aplasma sheath is formed near the upstream surface of the screen grid. Inthe single grid system of the present invention, the sheath is believedformed near the upstream surface of the insulating coating 18. Becauseof the high electrical resistance of the insulating coating 18, chargescan build up on its surface to form a virtual screen grid.

In this manner the plasma sheath is moved closer to the negativeaccelerator 14. This increases the field strength for a given voltagewhich, in turn, increases the ion beam current density.

The grid 14 is electrically conductive to establish the properelectrostatic field for accelerating the ions in the chamber 10. Becausea portion of the electrically conducting grid 14 is not covered by theinsulating material 18, sputtering erosion of the exposed surface of thegrid 14 maybe encountered. Where long life is of importance theelectrically conducting grid 14 is a metal which is resistant tosputtering erosion. Molybdenum is a grid metal that has been found to beresistant to such erosion.

The grid 14 is preferably of a metal which has a thermal expansioncoefficient that matches the thermal expansion coefiicient of theinsulating material 18, which is preferably glass. Molybdenum has beensatisfactory for grids 14 that are coated with glass. It is alsocontemplated that stainless steel, tungsten, and tantalum can be usedfor the grid 14.

A slurry is prepared by suspending finely milled particles of theinsulating material 18 in a solution of water or some other organic orinorganic solvent, This slurry is sprayed onto one face and walls of theholes in a perforated plate forming the grid 14. The grid surfaces to becoated are initially oxidized prior to being covered with the sprayedslurry.

The insulating coating 18 is preferably a type of glass that forms agood seal with the material of the grid 14. Corning glass 7052 has beenfound to be satisfactory for the slurry used in coating a molybdenumgrid.

When the sprayed on slurry dries, the grid 14 is placed in an oven whichis at a temperature hot enough to fuse the fine glass particles togetherand to seal the glass to the metal. Great care must be taken to insurethe proper firing temperature to prevent the glass from flowing over andcovering the accelerator grid holes or openings. If the temperature istoo low the glass will not fuse or form a good bond. The glasspreferably has a working temperature greater than about 1400 P so thatthe glass remains a good insulator at the ion thrustor operatingtemperature. Corning glass 7052 has a working temperature of of about2000 F.

After removing the grid from the firing oven it is permitted to coolslowly. FIG. 2 shows the glass coating 18 after firing. By using a fusedglass type seal, optimum results are obtained between the insulator andthe grid. The glass coating is permanently impervious to mercurydiffusion Which causes other types of oxides or ceramic coatings tofail.

While a preferred embodiment of the invention has been described it willbe appreciated that various modifications may be made to the disclosedstructure and materials without departing from the spirit of theinvention or the scope of the subjoined claims. For example, the size ofthe coated grid as well as the geometry of the perforations may bevaried. It is further contemplated that the geometry of the coating maybe changed to meet special requirements. A single grid acceleratorsystem constructed in accordance with the invention can be used withvarious types of thrustors using other propellant materials. By way ofexample, such an accelerator system can be used with a contactionization thrustor utilizing cesium as a propellant.

What is claimed is:

1. In an accelerator system for an ion thrustor having a chamber forcontaining an ionized propellant, the improvement comprising a singleelectrically conducted metal plate having a plurality of aperturestherein mounted at one end of said chamber, said plate having a firstsurface facing toward said chamber and a second surface facing away fromsaid chamber, each of said apertures having a wall extending betweensaid first surface and said second surface,

a fused glass coating covering said first surface and said wall of eachof said apertures for protecting the same from sputtering erosion, and

means for applying a potential to said metal plate that is highlynegative to said chamber whereby the ionized propellant is acceleratedthrough each of said apertures without contacting said wall thereof.

References Cited UNITED STATES PATENTS 3,117,416 1/1964 Harries 602023,156,090 11/1964 Kaufman 60202 3,177,654 4/1965 Gradecak 602023,354,644 11/1967 Moore 60202 3,452,237 6/ 1969 Wingerson 602D2X 2,998,523 8/1961 Muench et al -1 25084.5 3,015,032 12/1961 Hoyer et a131361X 3,359,733 12/1967 Forbes 60202 DOUGLAS HAIRT, Primary ExaminerUS. Cl. X.R.

